C.G.
Like the PP said, it is definitely not true that you will miscarry if you continue to breastfeed. Even if you had another issue, there is no reason to think breastfeeding would contribute to a miscarriage. I would ask your doctor why he said that as maybe he has some sort of other reasoning. He is doing his patients a disservice by giving out incorrect information.
You may find that pregnancy diminishes your milk supply due to the hormone shifts. Your son may chose to wean if he is not getting satisfied. The taste of your milk shouldn't be affected as pregnancy wouldn't change the composition. If he is only nursing at bedtime he may be perfectly content to continue. Another good site for breastfeeding info is www.askdrsears.com Good luck!